This invention relates to a mobile harvesting machine control system and more particularly to a control system for a power transmission assembly having a plurality of output modes for driving a functional element of the machine.
It is well known in harvesting machines to provide, within a total drive system, individual power transmission assemblies for particular functional elements of the machine, such as conveying elements. It is typical to drive such assemblies with a single input but to provide several or variable output modes including forward, neutral and reverse and perhaps variable speed. An example of the provision of such a tranmission is in the feed roll drive of a forage harvester where it is desirable to provide, in addition to the normal operating forward mode, reverse and neutral modes which are of particular value in clearing blockages from the feeding area of the machine.
Known control systems for shifting such transmissions have tended to be either costly, even though the transmissions themselves may be simple in construction, or, where attempts have been made to reduce cost, lacking in convenience and reliability. It is known, for example, to use a conventional double-acting hydraulic actuator, such as a double-acting piston-type cylinder, but the cylinder generally used is essentially an unbalanced actuator. The effective piston area on the rod side is less than on the other side of the piston so that, for a given hydraulic supply condition, speed and force of actuation is greater in one direction than in the other. This is in general inconvenient and requires more operator attention for precise control. Further, it is common for the hydraulic circuitry to be such that, after shifting, both sides of the hydraulic actuator are connected to a common portion of the hydraulic system, for example a return line to a reservoir. Thus, any change of pressure in that part of the hydraulic system is applied to both sides of the hydraulic actuator and in the case of an unbalanced actuator, such as a conventional double-acting cylinder, there will be a net unbalanced force in the actuator, possibly causing it to creep and shift the transmission from one mode to another. Such an unplanned change may be unsafe and is certainly inconvenient and may cause problems, such as serious plugging of a forage harvester header.